Distribution of light produced by a light source can be important or even critical in some applications. The light source can be, for example but not necessarily, a light emitting diode “LED”, a filament lamp, or a gas-discharge lamp. FIG. 1a shows a schematic illustration of a street lighting application where streetlamps 122 and 123 are arranged to illuminate a road 120. FIG. 1b shows a view of a section taken along the line A1-A1 shown in FIG. 1a, and FIG. 1c shows a view of a section taken along the line A2-A2 shown in FIG. 1a. Each of the streetlamps 122 and 123 may comprise, for example, a lighting fixture that comprises a plurality of light sources, e.g. light emitting diodes “LED”, and optical devices each of which being arranged to modify the light distribution pattern of one or more of the light sources. An exemplifying optical device 101 according to the prior art is illustrated in FIGS. 1e and 1f where FIG. 1f shows a view of a section taken along the line A-A shown in FIG. 1e. A light source 102 is arranged to radiate first light beams to a first geometric quarter-space 103 and second light beams to a second geometric quarter-space 104, where the first and second geometric quarter-spaces are defined by mutually perpendicular geometric planes 105 and 106 so that the geometric plane 105 constitutes a boundary between the first and second geometric quarter-spaces 103 and 104. In FIGS. 1e and 1f, some of the first light beams are depicted with dot-and-dash line arrows and some of the second light beams are depicted with dashed line arrows. It is to be noted that the above-mentioned geometric planes 105 and 106 are mere geometric concepts for illustrative purposes only but not physical elements of the optical device 101 or of the light source 102. The geometric plane 105 is parallel with the yz-plane of a coordinate system 199 and the geometric plane 106 is parallel with the xy-plane of the coordinate system 199. The optical device 101 comprises a lens-section 107 for modifying a light distribution pattern of the first light beams. The optical device 101 comprises a reflector surface 108 for reflecting at least a part of the second light beams to the first quarter-space 103 as illustrated in FIGS. 1e and 1f. The reflector surface 108 is a surface of a cavity 109. The geometric forms of the cavity 109 and the refractive index of the transparent material of the optical device 101 are selected so that the total reflection takes place on the reflector surface 108.
FIG. 1d shows polar plots illustrating simulated luminance distributions on the surface of the road 120 when optical devices of the kind described above are being used in an exemplifying situation where the distance D between the adjacent streetlamps is about 4.5 times the height H of streetlamp poles and the width W of a lane 121 is about a half of the height H of the streetlamp poles. The solid line polar plot shows the luminance distribution on the line A1-A1 shown in FIG. 1a and the dashed line polar plot shows the luminance distribution on the line A2-A2 that is on the middle of the lane 121. Angles φ1 and φ2 are defined in FIG. 1c and angles φ3 and φ4 are defined in FIG. 1b. An ideal situation would be such that the luminance is at a suitable level and uniform on the surface of the road. In FIG. 1d, a circle arc 124 illustrates a situation where the luminance is uniformly distributed.
It is inherent that it becomes more and more challenging to achieve a luminance distribution that is sufficiently uniform in the longitudinal direction of the road 120 when the distance D between adjacent streetlamps is increased. On the other hand, the costs of the street lighting can be reduced by increasing the distance D. Thus, there is a clear economic incentive to increase the distance D between adjacent streetlamps.